Geochemical Characteristics Of Rare Earth Elements In Karst Urban Groundwate

Rare earth elements（REEs）from anthropogenic sources have been found in rivers,lakes,oceans,groundwater,and even tap water and tap beverages all around the world since the 1990s.An increase in anthropogenic rare earth element concentrations in the aquatic environment has become a global problem.The research region for this study is Guiyang（a typical karst city）.A total of 52 samples of groundwater were taken.Inductively coupled plasma mass spectrometry was used to determine the contents of dissolved REEs（ICP-MS）.The basic hydrochemical features,the geochemical properties of REEs,and the distribution characteristics of anthropogenic REEs in groundwater in Guiyang are all included in this work.The findings suggest that groundwater quality of Guiyang city is generally good,which is slightly alkaline.The mass concentration of sulfate and nitrate in some samples exceeds the standard limit of class III groundwater,indicating that groundwater in Guiyang has been affected by human activities in different degrees.The main hydrochemical types of groundwater in Guiyang city are HCO₃·SO₄-Ca and HCO₃-Ca·Mg,and a small part are HCO₃-Ca·Mg and HCO₃-Ca.The total dissolved REE concentration（∑REE）in groundwater of Guiyang city is relatively low,ranging from31.89 ng/L to 71.54 ng/L.The REEs demonstrate that the urban area is greater than the suburban area,but there is no discernible difference across the entire region.Ln（CO₃）₂^-and Ln CO₃⁺are the main complex forms of REEs,and a small part are Ln³⁺and Ln SO₄⁺.The complexation of light,medium and heavy earth elements with different ions is different.The mass percentage of Ln³⁺and Ln SO₄⁺complexes decreased with the increase of the atomic number of REEs.The mass percentage of Ln（CO₃）₂^-and Ln CO₃⁺complexes increases with the increase of the atomic number of REEs.The（La/Yb）_SN of all groundwater samples is less than 1,indicating that heavy rare earth elements（HREE）is enriched relative to light rare earth elements（LREE）,while light LREE is relatively depleted.Meanwhile,all groundwater samples show Ce negative anomaly and Eu positive anomaly.There are positive anomalies of anthropogenic Gd in some groundwater of Guiyang city.The concentration of Gd_anth is up to 10.56 ng/L,and the proportion of Gd_anth is up to 71.6%in groundwater of Guiyang.In terms of region,the concentration sequence of Gd_anth in the six regions are Guanshanhu>Yunyan>Nanming>Baiyun>Wudang>Huaxi,which is basically consistent with the order of Gd concentration.Anthropogenic Gd is positively correlated with Cl^-,NO₃^-and Na⁺,which reflects the homology of human source Gd and urban wastewater in karst city groundwater.The results show that anthropogenic Gd has a good potential in detecting karst groundwater pollution and improving the recognition of karst groundwater system in urban areas.Therefore,strengthening the study of geochemical characteristics of rare earth elements in groundwater in karst areas will play a guiding role in judging the pollution of groundwater in karst cities and the wider and scientific use of rare earth elements in the future.In addition,anthropogenic Gd has a good potential in tracking karst groundwater pollution in urban areas and improving the recognition of karst groundwater system,which is of great scientific and practical significance for the study of groundwater pollution control in karst cities.